Accident Sequence
On August 9, 2017, at approximately 0730 central daylight time, an Air Tractor AT-802A airplane, registration N528MB, was substantially damaged during an accident near Galesburg, Illinois. The pilot, who was the sole occupant, was not injured. The airplane was being operated as a Title 14 Code of Federal Regulations Part 137 agricultural flight.
The pilot reported that the accident occurred about one hour into his first aerial-application flight of the day. He felt a slight "thud or bump," which he initially attributed to flying through his own propeller wash during a turn. Engine gauges indicated normal parameters as he continued to line up for another spray pass. Before the next pass, the engine experienced a similar "thud" that the pilot identified as a compressor stall. He entered a climb to gain safe altitude and maneuvered away from nearby homes. The engine experienced several more compressor stalls, and the pilot observed sparks exiting the engine exhaust ducts before a total loss of power. The pilot executed a forced landing in a nearby field, during which the airplane ground looped in dense soybean crop. The landing gear collapsed, and the outboard portion of the right wing and aileron were substantially damaged.
Engine Examination
Examination of the engine revealed that a single first-stage compressor rotor airfoil had fractured near the root. The fatigue crack originated from a thinned leading edge due to significant erosion on the bottom of the airfoil pressure side; however, the exact origin location could not be determined because of erosion damage. The fracture features were consistent with a crack that had progressed in high-cycle fatigue. Dimensional measurements of the fractured airfoil and the two adjacent airfoils showed they were up to 0.050 inch less than the minimum requirement for an airfoil. The airfoils also exhibited fine wear scratches, characteristic of impact by fine abrasive particles, and thinning of the leading edge. The remaining damage throughout the gas path was considered secondary to the separation of the single first-stage compressor rotor airfoil.
Maintenance History
According to maintenance documentation, the engine had accumulated 4,349.3 hours since new and 1,808.6 hours since being partially disassembled for inspection and repair on December 16, 2013. However, the logbook entry indicated that the gas generator was not inspected at that time. The most recent inspection of the gas generator was completed during an annual/100-hour inspection on May 1, 2017, about 99.3 hours before the accident. Based on available documentation, the engine had not been overhauled during its service history. The engine manufacturer recommends overhaul after 3,000 hours. Federal Aviation Administration regulations do not require Part 137 operators to comply with the manufacturer's recommended time between overhaul, provided the engine meets annual and 100-hour requirements.